1. Field of the Invention
The present invention relates to a method of machining the seats of a compressor piston used for a compressor of an air conditioning system (air conditioner) or the like.
2. Description of the Related Art
Control of the temperature, humidity, air flow and air purity in a vehicle passenger compartment, by an air conditioner, is necessary to provide a comfortable environment. The temperature control, etc. by the air conditioner is carried out mainly in such a manner that the compressor in a refrigerator (or a heat pump) circulates the working fluid and performs a refrigeration cycle (or a heat pump cycle).
Such a compressor is, for example, a swash plate type compressor 400, as shown in FIG. 9 (Japanese Unexamined Patent Publication No. 16-249140). The swash plate type compressor 400 is such that when a main drive shaft 416 is rotated by receiving power from a drive source, a swash plate 419 is moved and compressor pistons 450 (hereinafter called as xe2x80x9cpistonxe2x80x9d) are reciprocated so that the working fluid (refrigerant gas) is sucked, compressed and discharged. In the swash plate type compressor 400, hemispherical shoes 421 and 422 are interposed between the swash plate 419 and the seats 451 and 452 of the piston 450, so that the rotation and motion of the swash plate 419 is converted into the reciprocal motion of the pistons 450. The piston 450 includes hemispherical seats 451 and 452 having hemispherical recesses covering at least a portion of the hemispherical portions of the hemispherical shoes 421 and 422.
Incidentally, a method of machining the seats as described above is disclosed in the aforementioned Japanese Unexamined Patent Publication (Kokai) No. 6-249140 and Japanese Unexamined Patent Publication (Kokai) No. 10-220354. Also, though not disclosed in these publications, the periphery of the hemispherical recess has conventionally been chamfered. This chamfering has been carried out with a cutting tool having a large working diameter different from the cutting tool used for machining the hemispherical recess. This process is shown in FIG. 8 taking as an example the case in which the seat 452 of the piston 450 of the swash plate type compressor 400 is machined. According to this machining method, first, a cutting tool C1 having the working diameter R1 is rotated while being moved until the center of the cutting element thereof reaches the point P1 thereby to form a chamfered portion 452b. Next, the tool C1 is removed and a cutting tool C2 having the working diameter R2 (R2 less than R1) is moved until the center of the cutting element thereof reaches the point P2 thereby to form a hemispherical recess 452a. 
Since the chamfered portion and the recess of the seat are conventionally machined with different tools C1 and C2, loading and unloading of the workpiece (the piston or the stock thereof), mounting and demounting the workpiece and change or relocation of the tool consume extra time, thereby making it difficult to improve the machining efficiency of the seats and hence the production efficiency of the piston, although the situation may vary depending on the machining process and the machining equipment.
Also, mounting and demounting the work and changing of the tool makes it impossible to machine the chamfered portion and the recess under exactly the same conditions, and therefore, the machining precision of the depth of the spherical surface of the recess, for example, cannot be improved.
Further, in the case where the space in the work into which the tool is to be inserted is small, a large tool for machining the chamfered portion may not be inserted. Further, the provision of a specially relieved portion for inserting the tool to avoid this inconvenience undesirably limits the design latitude of the piston and the compressor.
Furthermore, a large working diameter of the tool reduces the degree of machining freedom, thereby sometimes making it difficult to adjust the chamferring angle and the chamfer width.
The present invention is made under these circumstances, and the object of the present invention is to provide a method of machining seats of a compressor piston by which a hemispherical recess and a chamfered portion on the periphery of the recess can be efficiently machined.
The inventors of the present invention have made vigorous research effort to solve the aforementioned problems and, by trial and error, reached the idea of machining and forming the recess and the chamfered portion of the seat of the compressor piston using the same tool, and developed a method of machining seats of the compressor piston according to the present invention.
Specifically, according to the present invention, there is provided a method of machining seats of a compressor piston, the seat having an approximately hemispherical recess adapted for sliding engagement with a hemispherical shoe of a compressor. The method comprises the steps of: annularly moving a cutting element along a surface of a piston while at least one of the cutting element and the piston is rotated with respect to the other to form an annularly chamfered portion in the surface; and moving the cutting element in direction toward the surface while the at least one of the cutting element and the piston is rotated with respect to the other to form a hemispherical recess in the surface.
In this method, the chamferring step and the recess forming step are carried out with the same cutting element and, therefore, the transfer of steps from the chamferring step to the recess forming step and from the recess forming step to the chamferring step can be smoothly carried out, thereby making it possible to reduce or eliminate the time required for loading and unloading the workpiece, mounting and demounting the workpiece and changing or relocating the tool. As a result, the efficiency of machining the seats of the compressor piston can be improved.
Also, since the piston seats are machined using the same cutting element for the chamferring step and the recess forming step without removing the workpiece, a position deviation and an offset of the center are not caused, thereby making it possible to improve the machining accuracy for the chamfered portion and the recess (such as the depth of the spherical surface of the recess).
Preferably, the step of annularly moving the cutting element is carried out prior to the step of moving the cutting element in a direction toward the surface.
By forming the recess after forming the annular cut portion becoming the chamfered portion, burrs are not formed by machining the hemispherical surface of the recess, and foreign matter can be prevented from entering in a gap between the recess and the shoe.
Preferably, the compressor piston comprises a piston of a swash plate type compressor and includes a hollow or solid cylindrical head portion, a neck portion extending from one side of the head portion, and two seats on the neck portion in opposed relation to each other and having approximately hemispherical recesses, the neck portion and the two seats constituting an approximately U-shaped space so that a swash plate can be arranged in the U-shaped space and shoes can be interposed between the swash plate and the seats to thereby enable the motion of the swash plate to produce a reciprocating motion of the compressor piston.
In this machining method, the chamferring step is carried out by moving the same cutting element as in the recess forming step along the annular path, and therefore as long as the recess forming step can be carried out by inserting the cutting tool carrying the cutting element in to a substantially U-shaped space, it is also possible to carry out the chamferring step in other words, even in the case where the two seats are oppositely arranged to form a substantially U-shaped space, the chamferring step can be carried out without interference between the cutting tool and the seats. As a result, the need of providing a special space for carrying out the chamferring step in the compressor piston is eliminated, thereby improving the freedom of designing the compressor piston.
Also, in view of the fact that the recesses of the two seats are arranged in an opposed relation to each other, the recesses and the chamfered portions of the two seats can be formed in a single step by machining the seats while rotating the compressor piston relative to the tool, thereby further improving the machining efficiency.
Preferably, the chamfered portion includes a first portion having a first chamfer width and a second portion having a second chamfer width different from the first chamfer width.
In the chamferring, a cutting tool with a large working diameter need not be used unlike in the prior art. Therefore, the width, the angle, the position, etc. of the chamfered portion can be easily controlled. For example, the chamfered portion is formed conveniently with a wide width portion and a narrow width portion.
By the way, the surface pressure acting on the recess of the seat can be controlled by the wide width portion and the narrow width portion. For example, the narrow width portion is formed in the area subjected to a large load so that the contact area between the shoes, and the recess is increased, thereby making it possible to reduce the surface pressure acting on them.
Thus, the narrow width portion is desirably formed in the area of the seat recess subjected to a large load (receiving a large surface pressure) in the compression stroke, while the wide width portion is desirably formed conversely in the area where the surface pressure between the seat and the shoe is small.
Also, the area formed with the wide width portion has a large gap between the shoe and the chamfered portion, with the result that the lubricant is increasingly guided and introduced into the particular portion for an improved slidability between the recess and the shoe.
Incidentally, the cutting tool may be a ball end mill or a cutting tool having a substantially spherical cutting element at the forward end thereof, or a cutting tool with a semicircular disk-shaped chip attached to the forward end of a shank. The cutting element may be a cutting edge or a grinding element. The machining work performed in the recess forming step and the chamferring step may be carried out either by rotating the cutting tool or by rotating the compressor piston. The two parts may be rotated relatively to each other. Similarly, the two parts may be moved relatively to each other along an annular path even in the case where the tool is moved along the annular path.
The machining method according to the present invention is applicable widely to the compressor piston including seats each having a substantially hemispherical recess and a chamfered portion on the periphery thereof as well as to the piston of a swash plate type compressor. The piston may be either single headed or double headed piston. Also, the compressor using the piston may be of either a variable displacement type or a constant displacement type. Further, the present invention is applicable to both the refrigeration cycle and the heat pump cycle. Furthermore, the present invention is applicable to a general type compressor as well as a vehicle air conditioner compressor.
The other objects and features will be understood from the following description of the preferred embodiments of the present invention.